Over the last ten years, direct sequence and spread spectrum (DS-SS) modulation systems have been increasing in importance.
At present, this technique is implemented not only in satellite navigation systems such as GPS and GLONASS, but it has also been introduced into terrestrial and satellite communications systems, e.g. US standard IS-95, GLOBALSTAR, and more recently in the third generation of mobile telephones using the UMTS standard, and also in the European satellite navigation system GALILEO.
The concept of DS-SS modulation, e.g. bi-binary phase shift keying (Bi-BPSK) introduces a pseudo-random noise (PRN) code which has the consequence of the resulting modulated signal presenting a passband that is wider than a signal that is transmitting only the data signals. It is in this sense that the spectrum density of the signal is said to be “spread”.
In the receiver, a locally-generated replica of the transmitted PRN code is aligned with the phase of the code of the received signal. In particular, in navigation receivers, code phase alignment is essential for determining accurately the time of arrival (TOA), which is used for determining the geometrical distance between the transmitter and the receiver. Once alignment has been achieved, it is possible to estimate carrier phase and to determine the symbols of the transmitted data.
This alignment is conventionally achieved in the receiver by means of a delay-locked loop (DLL), an example of which is described in the article by M. Simon et al. published in the work “Spread spectrum communications handbook” published by McGrawHill, Inc., 2nd edition, 1994.
Such alignment uses the result of correlation between the received signal and early and late versions (E and L) of a locally-generated reference code signal in order to calculate an error signal that is proportional to code phase error (the difference between the estimated code phase and the received phase).
This error signal must indicate the direction in which the phase of the reference signal needs to be offset (advanced or retarded) in order to be brought into synchronization with the received signal. The spacing between the early and late codes (E and L) is generally one bit of a pseudo-noise sequence known as a “chip”.
Signals of the square-root raised cosine (SRC) type (which have a raised cosine spectrum) are defined in the UMTS standard. The same type of SRC signal is likely also to be adopted in the above-mentioned GALILEO system. A digital receiver implementing such signals is described in the article by R. de Gaudezni et al. entitled “A digital chip timing recovery loop for band-limited direct-sequence spread-spectrum signals” published in IEEE Trans. on Comm., Vol. 41, No. 11, pp. 1760-1769, November 1993.
The accuracy with which time of arrival is measured is negatively disturbed by the presence of distortion due to multiple paths, and as a result, when performing telemetry, the precision with which position is determined is decreased, and when transmitting data, there is an increase in bit or frame error rate. This is particularly true when the multipath distortion is represented essentially by a single reflection coming from a point which is situated in the immediate environment of the receiver with a small dynamic range.
The superposition of the direct and reflected signals is thus liable to give rise to jitter which affects the TOA measurements performed by the DLL.
As a result, techniques that make it possible to reduce the impact of multiple paths on determining code phase are of very great interest, particularly in the field of navigation.
Until now, methods for compensating multiple paths for use in telemetry have been developed essentially in the context of GPS receivers.
As a result, a large number of those algorithms make use of the fact that the apparent chip rate of the publically available C/A code is much lower than the passband of the transmission. It is then advantageous to reduce the time differences between the early and late reference code signals E and L until they have a value that is less than one bit of a pseudo-random noise sequence (“one chip duration”) in order to reduce the error that is induced by the multipath beams.
In the context of SRC type systems, given that the frequency spectrum is strictly limited to (1+β) times the apparent code rate (where β designates the attenuation factor of SRC pulses), the above-mentioned methods are not effective in compensating multipath beams.
In the article by Philip G. Mattos entitled “Multipath elimination for the low-cost consumer GPS” published in the Proceedings of the ION GPS 1996 Conference in Kansas City, pp. 665-671, it has also been suggested to replace the early and late correlation points E and L by two early correlation points. However, that article does not give any means for implementing that technique.